Overall, the researchers found that people who consumed the most fish lowered their risk of depression by 17 percent compared to those who ate the least.

"Studies we reviewed indicated that high fish consumption can reduce the incidence of depression, which may indicate a potential causal relationship between fish consumption and depression," said lead researcher Fang Li, of the department of epidemiology and health statistics at the Medical College of Qingdao University in China.

But this association was only statistically significant for studies done in Europe, the researchers said. They didn't find the same benefit when they looked at studies done in North America, Asia, Australia or South America. The researchers don't know why the association was only significant for fish consumption in Europe.

The study was also only able to show an association between eating fish and the risk for depression, not that eating fish causes a lower risk for depression, Li said.

Still, Li thinks there may be reasons why fish may have an effect on depression.

"Fish is rich in multiple beneficial nutrients, including omega-3 fatty acids, high-quality protein, vitamins and minerals, which were associated with decreased risk of depression from our study," Li said.

The researchers pointed out that it's possible that the omega-3 fatty acids in fish may change the structure of brain membranes, or these acids may alter the way certain neurotransmitters work. Neurotransmitters are the brain's chemical messengers, sending information from brain cell to brain cell. Some neurotransmitters, such as dopamine and serotonin, are thought to be involved in depression, the researchers said.

The report was published Sept. 10 online in the Journal of Epidemiology & Community Health.

Depression affects 350 million people around the globe, according to background information in the study. The mood disorder is the leading cause of disability worldwide, according to the World Health Organization.

Past research has suggested that dietary factors may play a role in depression, the researchers said.

Continued

To look at the possible connection between eating fish and depression, Li and colleagues reviewed 26 studies published between 2001 and 2014. The studies included more than 150,000 people. Ten of the studies were done in Europe.

This process, called a meta-analysis, attempts to find consistent patterns across multiple studies.

In addition to an overall benefit from fish in curbing depression, Li's team found a difference between men and women. Specifically, the researchers found a slightly stronger association between eating a lot of fish and lowered depression risk in men by 20 percent. Among women, reduction in risk was 16 percent, the researchers said.

Simon Rego, director of psychology training at Montefiore Medical Center/Albert Einstein College of Medicine in New York City, said it's "impossible to draw any definitive conclusions about direct cause and effect" due to the study's design.

But, he added, "While the exact way fish may prevent depression is unknown, it's promising to learn that depression may be preventable for some people by making simple modifications to their lifestyle, such as by eating more fish."

Rego said it's especially important to look for novel treatments because depression can have a significant impact on people's lives, and many people don't respond fully to first-line depression treatments.

Future research needs to look into whether the effects of fish on depression vary by the type of fish eaten. In addition, this review didn't look at whether or not fish oil supplements could have the same effect.